Magnetic recording medium and magnetic storage apparatus

ABSTRACT

A magnetic recording medium includes a first magnetic layer, a second magnetic layer, and a non-magnetic coupling layer provided between the first and second magnetic layers so that the first and second magnetic layers are exchange-coupled and the magnetizations of the first and second magnetic layers are antiparallel. The first magnetic layer has an exchange coupling field Hex 1  which is larger than the respective coercivities Hc 1  and Hc 2  of the first and second magnetic layers.

BACKGROUND OF THE INVENTION

[0001] This application claims the benefit of a Japanese PatentApplication No.2001-272601 filed Sep. 7, 2001, the disclosure of whichis hereby incorporated by reference.

[0002] 1. Field of the Invention

[0003] The present invention generally relates to magnetic recordingmedia and magnetic storage apparatuses, and more particularly to amagnetic recording medium which is suited for high-density recording andcapable of carrying out high-speed recording and reproduction, and to amagnetic storage apparatus which uses such a magnetic recording medium.

[0004] 2. Description of the Related Art

[0005] Due to the developments in information processing technology,there are increased demands for high-density magnetic recording media.For example, for a hard disk, the magnetic recording media required tosatisfy such demands should include such characteristics as low noiseand improved thermal stability.

[0006] The recording density of longitudinal magnetic recording media,such as magnetic disks, has been increased considerably due to thereduction of medium noise and the development of magnetoresistive andhigh-sensitivity spin-valve heads. A typical magnetic recording mediumis comprised of a substrate, an underlayer, a magnetic layer, and aprotection layer which are successively stacked in this order. Theunderlayer is made of Cr or a Cr alloy, and the magnetic layer is madeof a Co alloy.

[0007] Various methods have been proposed to reduce the medium noise.For example, Okamoto et al., “Rigid Disk Medium For 5 Gbit/in²Recording”, AB-3, Intermag '96 Digest, proposes decreasing the grainsize and size distribution of the magnetic layer by reducing themagnetic layer thickness by the proper use of an underlayer made ofCrMo. U.S. Pat. No. 5,693,426 proposes the use of an underlayer made ofNiAl. Further, Hosoe et al., “Experimental Study of Thermal Decay inHigh-Density Magnetic Recording Media”, IEEE Trans. Magn. Vol.33, 1528(1997), for example, proposes the use of an underlayer made of CrTiB.The underlayers described above also promote c-axis orientation of themagnetic layer in a plane which increases the remanence magnetizationand the thermal stability of the written bits. In addition, proposalshave been made to reduce the thickness of the magnetic layer, toincrease the resolution or to decrease the width of the transitionbetween written bits. Furthermore, proposals have been made to decreasethe exchange coupling between grains by promoting more Cr segregation ina magnetic layer which is made of the CoCr alloy.

[0008] However, as the grains of the magnetic layer become smaller andmore magnetically isolated from each other, the written bits becomeunstable due to thermal activation and to demagnetizing fields whichincrease with linear density. Lu et al., “Thermal Instability at 10Gbit/in² Magnetic Recording”, IEEE Trans. Magn. Vol.30, 4230 (1994)demonstrated, by micromagnetic simulation, that exchange-decoupledgrains having a diameter of 10 nm and the ratio K_(u)V/k_(B)T˜60 in 400kfci di-bits are susceptible to significant thermal decay, where K_(u)denotes the magnetic anisotropy constant, V denotes the average magneticgrain volume, k_(B) denotes the Boltzmann constant, and T denotes thetemperature. The ratio K_(u)V/k_(B)T is also referred to as a thermalstability factor.

[0009] It has been reported in Abarra et al., “Thermal Stability ofNarrow Track Bits in a 5 Gbit/in² Medium”, IEEE Trans. Magn. Vol.33,2995 (1997), that the presence of intergranular exchange interactionstabilizes written bits, as demonstrated by MFM studies of annealed 200kfci bits on a 5 Gbit/in² CoCrPtTa/CrMo medium. However, more graindecoupling is essential for recording densities of 20 Gbit/in² orgreater.

[0010] The obvious solution has been to increase the magnetic anisotropyof the magnetic layer. But unfortunately, the increased magneticanisotropy places a great demand on the head write field which degradesthe “overwrite”, performance, which is the ability to write overpreviously written data.

[0011] In addition, the coercivity of thermally unstable magneticrecording medium increases rapidly with decreasing switching time, asreported in He et al., “High Speed Switching in Magnetic RecordingMedia”, J. Magn. Magn. Mater. Vol.155, 6 (1996), for magnetic tapemedia, and in J. H. Richter, “Dynamic Coervicity Effects in Thin FilmMedia”, IEEE Trans. Magn. Vol.34, 1540 (1997), for magnetic disk media.Consequently, adverse effects are introduced in the data rate, that is,how fast data can be written on the magnetic layer and the amount ofhead field required to reverse the magnetic grains.

[0012] On the other hand, another proposed method of improving thethermal stability increases the orientation ratio of the magnetic layerby appropriately texturing the substrate under the magnetic layer. Forexample, Akimoto et al., “Relationship Between Magnetic CircumferentialOrientation and Magnetic Thermal Stability”, J. Magn. Magn. Mater.(1999), in press, report through micromagnetic simulation that theeffective ratio K_(u)V/k_(B)T is enhanced by a slight increase in theorientation ratio. This further results in a weaker time dependence forthe coercivity which improves the overwrite performance of the magneticrecording medium, as reported in Abarra et al., “The Effect ofOrientation Ratio on the Dynamic Coercivity of Media for >15 Gbit/in²Recording”, EB-02, Intermag '99, Korea.

[0013] Furthermore, keepered magnetic recording media have been proposedfor thermal stability improvement. The keeper layer is made up of amagnetically soft layer that is parallel to the magnetic layer. Thissoft layer can be disposed either above or below the magnetic layer.Oftentimes, a Cr isolation layer is interposed between the soft layerand the magnetic layer. The soft layer reduces the demagnetizing fieldsin the written bits on the magnetic layer. However, coupling themagnetic layer to a continuously-exchanged coupled soft layer defeatsthe purpose of decoupling the grains of the magnetic layer. As a result,the medium noise increases.

[0014] In order to improve the thermal stability and to reduce themedium noise, magnetic recording media and magnetic storage apparatuseshave been proposed in U.S. patent application Ser. No. 09/425,788 filedOct. 22, 1999, which is incorporated herein by reference, and in whichthe assignee is the same as the assignee of this application. Thispreviously proposed magnetic recording medium is comprised of at leastone exchange layer structure, and a magnetic layer formed on theexchange layer structure, wherein the exchange layer structure includesa ferromagnetic layer and a non-magnetic coupling layer provided on theferromagnetic layer and under the magnetic layer, and the ferromagneticlayer and the magnetic layer have antiparallel magnetizations. Accordingto this previously proposed magnetic recording medium, it is possible toimprove the thermal stability of the written bits, reduce the mediumnoise, and realize a high-density recording having a high reliabilitywithout adversely affecting the performance of the magnetic recordingmedium.

[0015] In other words, in this previously proposed magnetic recordingmedium, the non-magnetic coupling layer (or the non-magnetic exchangelayer) is interposed between the ferromagnetic layer that forms a firstmagnetic layer and the magnetic layer that forms a second magneticlayer. When the structure includes first and second magnetic layershaving antiparallel magnetizations, the first and second magnetic layersmutually cancel portions of the magnetizations. Hence, it is possible toincrease the effective grain size of the magnetic layer withoutsubstantially affecting the resolution. Therefore, from the point ofview of the grain volume, it is possible to increase the apparentthickness of the magnetic layer so as to realize a magnetic recordingmedium having a good thermal stability.

[0016] Accordingly, this previously proposed magnetic recording mediumemploys a basic structure made up of the ferromagnetic layer (the firstmagnetic layer) and the magnetic layer (the second magnetic layer), soas to improve the thermal stability and to reduce the medium noise.

[0017] When an external recording magnetic field is applied to thispreviously proposed magnetic recording medium, the first and secondmagnetic layers first assume parallel magnetizations, and when therecording magnetic field decreases to zero (residual magnetizationstate) thereafter, the magnetization of the first magnetic layer isswitched and becomes antiparallel to the magnetization of the secondmagnetic layer.

[0018] However, as the recording density and the signal transfer rateincrease, it becomes necessary to also increase the recording andreproducing speed. For this reason, the need to wait for the switchingof the magnetization to occur in the first magnetic layer afterrecording may interfere with the realization of high-speed recording andreproduction.

[0019] In other words, the first and second magnetic layers of thispreviously proposed magnetic recording medium assume antiparallelmagnetizations in the residual magnetization state, and when theexternal recording magnetic field is applied in this state, the firstand second magnetic layers assume parallel magnetizations. Then, whenthe recording magnetic field thereafter decreases to zero to assume theresidual magnetization state once again, the magnetization of the firstmagnetic layer is switched to become antiparallel to the magnetizationof the second magnetic layer. During this process, it is necessary towait for the first magnetic layer to naturally make the magnetizationswitch.

[0020] But when the recording speed is increased and recording to anadjacent bit is made before the first magnetic layer makes themagnetization switch, the position of the bit which is to be recordedmay shift due to a counter magnetic field from the bit in the parallelmagnetization state. In this case, a non-linear transition shift (NLTS)deteriorates, and adversely affects the recording.

[0021] On the other hand, when measures are taken to reduce the timefrom recording to reproduction, an abnormal signal is generated toprevent normal reproduction if the reproduction is carried out beforethe first magnetic layer is switched to the antiparallel magnetizationstate from the parallel magnetization state.

SUMMARY OF THE INVENTION

[0022] Accordingly, it is a general object of the present invention toprovide a novel and useful magnetic recording medium and magneticstorage apparatus, in which the problems described above are eliminated.

[0023] Another and more specific object of the present invention is toprovide a magnetic recording medium which has first and second magneticlayers with antiparallel magnetizations to realize improved thermalstability and reduced medium noise, and that is capable of carrying outmagnetic recording and reproduction at a high speed, and to provide amagnetic storage apparatus which employs such a magnetic recordingmedium.

[0024] Still another object of the present invention is to provide amagnetic recording medium comprising a first magnetic layer, a secondmagnetic layer, and a non-magnetic coupling layer provided between thefirst and second magnetic layers so that the first and second magneticlayers are exchange-coupled and magnetizations of the first and secondmagnetic layers are antiparallel, where the first magnetic layer has anexchange coupling field Hex1 which is larger than respectivecoercivities Hc1 and Hc2 of the first and second magnetic layers.According to the magnetic recording medium of the present invention, themagnetizations of the first and second magnetic layers can be maintainedantiparallel in a residual magnetization state, and it is possible torealize a high recording density and high-speed recording andreproduction.

[0025] A switching field Hsw* which switches the magnetization of thefirst magnetic layer to become parallel to the magnetization of thesecond magnetic layer may be set to a sum of the exchange coupling fieldHex1 and the coercivity Hc1 of the first magnetic layer. In this case,it is possible to set a recording field within a range which does notreach the level of the switching field Hsw*, so that it is possible topositively realize a magnetic recording medium in which themagnetizations of the first and second magnetic layers are rotated whilemaintaining antiparallel magnetizations of the first and second magneticlayers.

[0026] A magnetization and thickness product t1Ms1 of the first magneticlayer is preferably smaller than a magnetization and thickness productt2Ms2 of the second magnetic layer, where t1 denotes a thickness of thefirst magnetic layer, Ms1 denotes a magnetization of the first magneticlayer, t2 denotes a thickness of the second magnetic layer, and Ms2denotes a magnetization of the second magnetic layer. In this case, itis possible to increase the exchange coupling field Hex1 of the firstmagnetic layer having a small magnetization and thickness product t1Ms1,so that it is possible to more positively realize a magnetic recordingmedium in which the exchange coupling field Hex1 is larger than thecoercivities Hc1 and Hc2 of the first and second magnetic layers.

[0027] The coercivity Hc1 of the first magnetic recording medium ispreferably smaller than the coercivity Hc2 of the second magneticrecording medium. In this case, it is possible to determine a main-subrelationship of the first and second magnetic layers. In other words, itis possible to design a magnetic recording medium in which the secondmagnetic layer, which is set to have the large coercivity Hc2, is usedas the main recording layer.

[0028] The magnetic recording medium may further comprise a couplingintensifying region, provided near the boundary of the non-magneticcoupling layer and at least one of the first and second magnetic layers,for intensifying the exchange coupling strength between the first andsecond magnetic layers. Further, the coupling intensifying region may bemade of a material selected from a group consisting of Fe, Co, Ni andalloys thereof. With the coupling intensifying region, it is possible toobtain an exchange coupling field Hex which further increases theexchange coupling between the first and second magnetic layers.

[0029] A further object of the present invention is to provide apatterned medium comprising a recording surface, and a plurality of unitrecording portions, provided on the recording surface, having boundarieswhich are separated among adjacent unit recording portions. Each of theplurality of unit recording portions preferably has a stacked structurecomprising a first magnetic layer, a second magnetic layer, and anon-magnetic coupling layer provided between the first and secondmagnetic layers so that the first and second magnetic layers areexchange-coupled and magnetizations of the first and second magneticlayers are antiparallel, where the first magnetic layer has an exchangecoupling field Hex1 which is larger than respective coercivities Hc1 andHc2 of the first and second magnetic layers. According to the patternedmedium of the present invention, it is possible to realize a highrecording density and high-speed recording and reproduction.

[0030] Another object of the present invention is to provide a magneticstorage apparatus comprising at least one magnetic recording medium, andat least one head for applying a field to the magnetic recording medium,where the magnetic recording medium comprises a first magnetic layer, asecond magnetic layer, and a non-magnetic coupling layer providedbetween the first and second magnetic layers so that the first andsecond magnetic layers are exchange-coupled and magnetizations of thefirst and second magnetic layers are antiparallel, and the firstmagnetic layer has an exchange coupling field Hex1 which is larger thanrespective coercivities Hc1 and Hc2 of the first and second magneticlayers. According to the magnetic storage apparatus of the presentinvention, it is possible to realize high recording density andhigh-speed recording and reproduction.

[0031] The field from the head may be larger than a coercivity Hc2 ofthe second magnetic layer and smaller than a switching field Hsw* whichswitches the magnetization of the first magnetic layer to becomeparallel to the magnetization of the second magnetic layer. Moreover,the switching field Hsw* may be set to the sum of the exchange couplingfield Hex1 and the coercivity Hc1 of the first magnetic layer. In thesecases, it is possible to positively realize the high-speed recording.

[0032] Other objects and further features of the present invention willbe apparent from the following detailed description when read inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033]FIG. 1 is a cross-sectional view showing the main parts of oneembodiment of a magnetic recording medium according to the presentinvention;

[0034]FIG. 2 is an enlarged cross-sectional view showing the main partsof a modification of the FIG. 1 embodiment of the magnetic recordingmedium;

[0035]FIG. 3 is a diagram showing a hysteresis loop of the FIG. 2modification of the magnetic recording medium;

[0036]FIGS. 4A and 4B respectively are diagrams showing switching of themagnetizations in the FIG. 2 modification of the magnetic recordingmedium and a previously proposed magnetic recording medium;

[0037]FIG. 5 is a diagram showing a portion of a recording surface of apatterned medium on an enlarged scale;

[0038]FIG. 6 is a cross-sectional view showing the main parts of oneembodiment of a magnetic storage apparatus according to the presentinvention; and

[0039]FIG. 7 is a plan view showing the main parts of the magneticstorage apparatus shown in FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0040]FIG. 1 is a cross-sectional view showing the main parts of oneembodiment of a magnetic recording medium according to the presentinvention. A magnetic recording medium 10 shown in FIG. 1 includes anon-magnetic substrate 11, a seed layer 12, an underlayer 13, anon-magnetic intermediate layer 14, a first magnetic layer 15, anon-magnetic coupling layer 16, a second magnetic layer 17, and aprotection layer 18 which are successively stacked in this order. Themagnetic recording medium 10 can be produced by sputtering, for example.A lubricant layer 19 may further be provided on top, of the protectionlayer 18.

[0041] The non-magnetic substrate 11 is made of for example, Al, glassor Si. The non-magnetic substrate 11 may be mechanically textured, ifdesired, but such texturing is not required.

[0042] The seed layer 12 may be made of NiP or NiAl, for example, butthe seed layer 12 is preferably made of NiP, for example, especially inthe case where the non-magnetic substrate 11 is made of Al or an Alalloy. The seed layer 12 may or may not be oxidized, and may or may notbe mechanically textured. The seed layer 12 may be made of a B2structure alloy such as NiAl and FeAl when the non-magnetic substrate 11is made of glass, for example. The seed layer 12 is provided to promotea (001) or (112) texture of the underlayer 13 which is formed on theseed layer 12. The underlayer 13 may be made of Cr or a Cr alloy,similarly as in the case of a conventional magnetic recording medium.

[0043] In a case where the magnetic recording medium 10 is a magneticdisk, the mechanical texturing provided on the non-magnetic substrate 11or the seed layer 12 which is made of NiP is made in a circumferentialdirection of the magnetic disk, that is, in the direction in which thetracks of the magnetic disk extend.

[0044] The non-magnetic intermediate layer 14 is provided to furtherpromote epitaxy, narrow the grain distribution of the first magneticlayer 15, and orient the anisotropy axes (axes of easy magnetization) ofthe first magnetic layer 15 along a plane parallel to the recordingsurface of the magnetic recording medium 10. The non-magneticintermediate layer 14 is made of an hcp structure alloy such as CoCr-M,where M=B, Mo, Nb, Ta, W, Cu or alloys thereof, and has a thickness in arange of 1 to 5 nm.

[0045] The first magnetic layer 15 is made of a material such as Co, Ni,Fe, Co alloy, Ni alloy or the like. In other words, Co alloys such asCoCr, CoCrTa, CoCrPt and CoCrPt-M, where M=B, Mo, Nb, Ta, W, Cu oralloys thereof, may be used for the first magnetic layer 15. Especiallywhen using a Co alloy for the first magnetic layer 15, the Coconcentration of the Co alloy may be set high, that is, the Co alloy maybe Co rich, so as to increase the exchange coupling magnetic field(hereinafter simply referred to as an exchange coupling field), whichwill be described later. The first magnetic layer 15 preferably has athickness in the range of 2 to 30 nm, for example.

[0046] The non-magnetic coupling layer 16 is made of a material such asRu, Rh, Re, Ir, Cr, Cu, Ru alloy, Rh alloy, Re alloy, Ir alloy, Cralloy, Cu alloy or alloys thereof. For example, when the non-magneticcoupling layer 16 is made of Ru, the non-magnetic coupling layer 16 hasa thickness in the range of 0.4 to 1.0 nm, and desirably has a thicknesson the order of approximately 0.8 nm. For this particular thicknessrange of the non-magnetic coupling layer 16, the magnetizations of thefirst magnetic layer 15 and the second magnetic layer 17 (which will bedescribed later) are antiparallel.

[0047] The second magnetic layer 17 is made of a material such as Co ora Co alloy such as CoCr, CoCrTa, CoCrPt, CoCrPt-M, where M=B, Mo, Nb,Ta, W, Cu or alloys thereof. Especially when using a Co alloy for thesecond magnetic layer 17, the Co concentration of the Co alloy may beset high, that is, the Co alloy may be Co rich, so as to make theexchange coupling field large. For example, the second magnetic layer 17preferably has a thickness in the range of 2 to 30 nm. Of course, thelayer structure of the second magnetic layer 17 is not limited to asingle-layer structure, and the second magnetic layer 17 may employ amulti-layer structure.

[0048] The protection layer 18 may be made of C, for example. Inaddition, the lubricant layer 19 is preferably made of an organiclubricant, for example, for use with a magnetic transducer such as aspin-valve head. The protection layer 18 and the lubricant layer 19 forma protection layer structure at the recording surface of the magneticrecording medium 10.

[0049] Obviously, the layer structure under the exchange layer structureis not limited to that shown in FIG. 1. For example, the underlayer 13may be made of Cr or a Cr alloy and formed to a thickness in the rangeof 5 to 40 nm on the non-magnetic substrate 11, and the first magneticlayer 15 may be provided on this underlayer 13. In addition, althoughthe first and second magnetic layers 15 and 17 having the antiparallelmagnetizations are respectively formed by one magnetic layer each inthis embodiment, it is possible, for example, to additionally provideunder the first magnetic layer 15 one or more magnetic layers havingantiparallel magnetization with respect to an adjacent magnetic layer.In this case, an exchange coupling field Hex of each additionallyprovided magnetic layer is set larger than a coercivity Hc2 of thesecond magnetic layer 17, so that the magnetizations (magnetizationdirections) of each additionally provided magnetic layer rotate togetherwith the first and second magnetic layers 15 and 17.

[0050] The magnetic recording medium 10 having the basic structuredescribed above is characterized in that the first and second magneticlayers 15 and 17 maintain the antiparallel magnetization states at thetime of recording, and the magnetization directions of the first andsecond magnetic layers 15 and 17 rotate together. For this reason, it isdesirable that the recording magnetic field (hereinafter simply referredto as a recording field) that is applied to the magnetic recordingmedium 10 is in a range that does not create a switching magnetic field(hereinafter simply referred to as a switching field) Hsw* that acts toswitch the magnetization of the first magnetic layer 15 to becomeparallel to the magnetization of the second magnetic layer 17. Theposition of the switching field Hsw* can be found from a coercivity Hclof the first magnetic layer 15 and an exchange coupling field Hex1 ofthe first magnetic layer 15 which is generated by the exchange couplingof the first and second magnetic layers 15 and 17, as will be describedlater in more detail.

[0051] The exchange coupling field Hex is the field which is generatedby the exchange coupling of the first and second magnetic layers 15 and17. Generally, the exchange coupling field Hex1 of the first magneticlayer 15 can be obtained from Hex1=J/t1Ms1, where J1 denotes an exchangecoupling constant, t1 denotes a thickness of the first magnetic layer15, and Ms1 denotes a magnetization of the first magnetic layer 15.Similarly, an exchange coupling field Hex2 of the second magnetic layer17 can be obtained from Hex2=J/t2Ms2, where J denotes the exchangecoupling constant, t2 denotes a thickness of the second magnetic layer17, and Ms2 denotes a magnetization of the second magnetic layer 17. Inthis specification, a description will be given by focusing on theexchange coupling field Hex1 generated in the first magnetic layer 15.

[0052] When the exchange coupling field Hex1 is set to be larger thanboth the coercivity Hc1 of the first magnetic layer 15 and thecoercivity Hc2 of the second magnetic layer 17, it is possible to makethe magnetizations of the first and second magnetic layers 15 and 17mutually antiparallel. In addition, because the desired switching fieldHsw* can be obtained from the sum of the exchange coupling field Hex1and the coercivity Hc1 of the first magnetic layer 15, as will bedescribed later, it is possible to carry out the recording whilemaintaining the magnetizations of the first and second magnetic layers15 and 17 in an antiparallel state by applying on the magnetic recordingmedium 10 a recording field which does not reach the level of theswitching field Hsw*.

[0053] Furthermore, when the coercivity Hc1 of the first magnetic layer15 is set to be large, the difference between the coercivity Hc2 of thesecond magnetic layer 17 and the switching field Hsw* can be made large,to thereby enable an increased degree of freedom of design of themagnetic recording medium 10.

[0054] In this specification, the switching field Hsw* refers to thefield which switches the magnetization of the first magnetic layer 15 tobecome parallel to the magnetization of the second magnetic layer 17when an external field is applied to the magnetic recording medium 10while increasing the field strength, in a state where the coercivity Hc2of the second magnetic layer 17 is smaller than the exchange couplingfield Hex1.

[0055] Next, a more detailed description will be given of thecharacterizing structures described above which are included in themagnetic recording medium 10.

[0056] In this embodiment, the coercivity Hc2 of the second magneticlayer 17 is set to approximately 4 kOe, and the coercivity Hc1 of thefirst magnetic layer 15 is set to approximately 0.5 kOe, for example.Hence, the coercivity Hc2 of the second magnetic layer 17 issufficiently large compared to the coercivity Hc1 of the first magneticlayer 15.

[0057] The magnetization and thickness product t2Ms2 of the secondmagnetic layer 17 is set to be larger than the magnetization andthickness product t1Ms1 of the first magnetic layer 15. For this reason,the difference that is obtained by subtracting the magnetization andthickness product t1Ms1 of the first magnetic layer 15 from themagnetization and thickness product t2Ms2 of the second magnetic layer17 mainly contributes to the signal at the time of the reproduction. Inaddition, since the magnetization and thickness product t1Ms1 of thefirst magnetic layer 15 is set to be small, the exchange coupling fieldHex1 can be made large, because Hex1=J/t1Ms1 as described above.

[0058] Furthermore, in the magnetic recording medium 10 of thisembodiment, it is desirable to provide a coupling intensifying regionfor intensifying the exchange coupling strength between the secondmagnetic layer 17 and the first magnetic layer 15, in addition to thebasic structure shown in FIG. 1.

[0059]FIG. 2 is an enlarged cross-sectional view showing a portion of amodification of the FIG. 1 embodiment of the magnetic recording medium10 that includes the coupling intensifying region. More particularly,FIG. 2 shows the layer structure of the part of this modification of themagnetic recording medium 10, including a coupling intensifying regionprovided between the non-magnetic coupling layer 16 and the first andsecond magnetic layers 15 and 17.

[0060] In the layer structure shown in FIG. 2, a lower couplingintensifying region 21 is provided between the first magnetic layer 15and the non-magnetic coupling layer 16, and an upper couplingintensifying region 22 is provided between the non-magnetic couplinglayer 16 and the second magnetic layer 17. However, it is not essentialto provide both the upper and lower coupling intensifying regions 21 and22, and only one of the upper and lower coupling intensifying regions 21and 22 may be provided. The magnetization of the lower couplingintensifying region 21 is parallel to the magnetization of the firstmagnetic layer 15, and the magnetization of the upper couplingintensifying region 22 is parallel to the magnetization of the secondmagnetic layer 17. The lower coupling intensifying region 21, togetherwith the first magnetic layer 15, has a function of intensifying theexchange coupling between the first and second magnetic layers 15 and17. Similarly, the upper coupling intensifying region 22, together withthe second magnetic layer 17, has a function of intensifying theexchange coupling between the first and second magnetic layers 15 and17. The exchange coupling between the first and second magnetic layers15 and 17 can be intensified even when only one of the upper and lowercoupling intensifying regions 22 and 21 is provided.

[0061] The lower coupling intensifying region 21 may be formed as aportion of either the first magnetic layer 15 or the non-magneticcoupling layer 16, or it may be formed as an interface on the surface ofthe first magnetic layer 15 or on the non-magnetic coupling layer 16. Inaddition, the lower coupling intensifying region 21 may be formed as afull layer with a relatively uniform thickness or it may be formed as aseries of projections. Similarly, the upper coupling intensifying region22 may be formed as a portion of either the second magnetic layer 17 orthe non-magnetic coupling layer 16, or it may be formed as an interfaceon the surface of the second magnetic layer 17 or the non-magneticcoupling layer 16. Further, the upper coupling intensifying region 22may be formed as a full layer with a relatively uniform thickness or itmay be formed as a series of projections.

[0062] The upper and lower coupling intensifying regions 22 and 21 arepreferably made of Fe, Co, Ni or alloys thereof. It is particularlydesirable to use materials such as Co, CoCr and CoCrTa for the upper andlower coupling intensifying regions 22 and 21. Moreover, the upper andlower coupling intensifying regions 22 and 21 may also be made of Co—X,CoCr—Y or CoCrTa—Y, where X=Pt, Ta, B, Cu, W, Mo, Nb, Ru, Rh, Ir oralloys thereof, and Y=Pt, B, Cu, W, Mo, Nb, Ru, Rh, Ir or alloysthereof.

[0063] It is desirable that the maximum thickness of the materialforming each of the upper and lower coupling intensifying regions 22 and21 is limited to approximately 2 nm. In addition, the material formingeach of the upper and lower coupling intensifying regions 22 and 21 mayexist in a surface state or in a dispersed state. For example, thefunction of intensifying the exchange coupling strength is sufficientlydisplayed even in a state where a desired material used is dispersed ina granular state within or on the surface of the first magnetic layer15, for example. Accordingly, even in a state where only a small amountof the desired material is dispersed within or on the surface of thefirst magnetic layer 15, for example, the dispersed material as a wholecan sufficiently function as a coupling intensifying region.

[0064] The thickness of the desired material within each of the upperand lower coupling intensifying regions 22 and 21 is approximately 2.0nm or less. Because the characteristics required of the magneticrecording medium 10 change depending on the material that is used toform the upper and lower coupling intensifying regions 22 and 21, it isdesirable to determine the thickness of the material forming each of theupper and lower coupling intensifying regions 22 and 21 by taking suchfactors into consideration.

[0065] The materials such as Fe, Co, Ni and alloys thereof, which aresuited for forming the upper and lower coupling intensifying regions 22and 21, may also be used to form the first and second magnetic layers 15and 17. Hence, the composition of the material forming the upper andlower coupling intensifying regions 22 and 21 may be the same as, orsimilar to, the composition of the material forming the upper and lowermagnetic layers 15 and 17. However, it is desirable that the materialforming the upper and lower coupling intensifying regions 22 and 21 isricher in Co (or the like) compared to the material forming the firstand second magnetic layers 15 and 17. For example, compared to materialswhich include Co and are generally used to form a magnetic layer, it isdesirable that the Co-content of the material forming the upper andlower coupling intensifying regions 22 and 21 is at least 10 at % to 20at % richer. Therefore, even in a case where materials having similarcompositions are used for the upper and lower coupling intensifyingregions 22 and 21 and the first and second magnetic layers 15 and 17,the upper and lower coupling intensifying regions 22 and 21 are Co richcompared to the first and second magnetic layers 15 and 17.

[0066] The materials described above which are rich in Co (or the like)may also be used to form the first and second magnetic layers 15 and 17.In this case, the lower coupling intensifying region 21 is included inthe first magnetic layer 15, and the surface of the first magnetic layer15 (that is, the interface between the first magnetic layer 15 and thenon-magnetic coupling layer 16) substantially corresponds to the lowercoupling intensifying region 21. In addition, the upper couplingintensifying region 22 is included in the second magnetic layer 17, andthe surface of the second magnetic layer 17 (that is, the interfacebetween the second magnetic layer 17 and the non-magnetic coupling layer16) substantially corresponds to the upper coupling intensifying region22. Hence, it is unnecessary in this case to prepare a material forseparately forming the upper and lower coupling intensifying regions 22and 21.

[0067] Accordingly, the lower coupling intensifying region 21 simplyneeds to exist substantially at a boundary of the first magnetic layer15 and the non-magnetic coupling layer 16, and the upper couplingintensifying region 22 simply needs to exist substantially at a boundaryof the second magnetic layer 17 and the non-magnetic coupling layer 16.

[0068] In this modification, the upper and lower coupling intensifyingregions 22 and 21 are preferably respectively made of Co having athickness of 1 nm. By employing the layer structure which includes theupper and lower coupling intensifying regions 22 and 21, the exchangecoupling strength between the first and second magnetic layers 15 and 17is increased. In addition, among the coercivity Hc2 of the secondmagnetic layer 17, the coercivity Hc1 of the first magnetic layer 15 andthe exchange coupling field Hex1 of the first magnetic layer 15, boththe coercivities Hc1 and Hc2 are smaller than the exchange couplingfield Hex1. According to this layer structure, the coercivity Hc12 isnaturally smaller than the switching field Hsw*.

[0069]FIG. 3 is a diagram showing a hysteresis loop of the FIG. 2modification of the magnetic recording medium 10. More particularly,FIG. 3 shows the hysteresis loop in which the abscissa indicates thefield and the ordinate indicates the Kerr signal due to the Kerr effect.It should be noted that the hysteresis loop of the FIG. 1 embodimentwill be of a similar shape to that shown in FIG. 3, except that thevarious parameters will be somewhat shifted.

[0070] Arrows ST1 through ST4 indicated in the upper part of FIG. 3respectively indicate magnetization states (i.e., the states of thedirection of magnetization) of the first and second magnetic layers 15and 17. The hysteresis loop shown in FIG. 3 includes a main hysteresisloop MAR at a central portion, and a sub-hysteresis loop SUR on both theright and left portions.

[0071] The large main hysteresis loop MAR is shown for a case where themagnetizations of the first and second magnetic layers 15 and 17 rotatetogether while maintaining the antiparallel state, that is, for a casewhere the state ST2 and the state ST3 are repeated.

[0072] On the other hand, the small sub-hysteresis loop SUR on the rightshows a case where the magnetization of the first magnetic layer 15switches from the antiparallel state to the parallel state with respectto the magnetization of the second magnetic layer 17, and vice versa. InFIG. 3, y indicates a position of the switching field Hsw* where themagnetization of the first magnetic layer switches from the antiparallelstate to the parallel state with respect to the magnetization of thesecond magnetic layer 17.

[0073] The sub-hysteresis loop SUR may be regarded as a hysteresis loop(minor loop) indicating the magnetization state of the first magneticlayer 15. In other words, when a field is applied in a positivedirection (+10 kOe) from a state (residual magnetization state)indicated by δ, the sub-hysteresis loop SUR passes the position γ andfollows SUR-1 on the right side. In this state, the magnetization of thefirst magnetic layer 15 switches from the state ST3, which isantiparallel to the magnetization of the second magnetic layer 17, tothe state ST4, which is parallel to the magnetization of the secondmagnetic layer 17. On the other hand, when the field is reduced from thestate ST4 (i.e., is reduced by 10 kOe), the sub-hysteresis loop SURfollows SUR-2 on the left side. In this state, the magnetization of thefirst magnetic layer 15 switches from the state ST4 which is parallel tothe magnetization of the second magnetic layer 17 to the state ST3 whichis antiparallel to the magnetization of the second magnetic layer 17.

[0074] Therefore, as may be seen from FIG. 3, the magnetizations of thefirst and second magnetic layers 15 and 17 can be maintained in theantiparallel state when the recording field is applied in a range of themain hysteresis loop MAR in which the applied field is smaller than theswitching field Hsw*, as indicated by γ.

[0075] The approximate center of the sub-hysteresis loop SUR indicatesthe exchange coupling field Hex1 of the first magnetic layer 15. Inaddition, in the main hysteresis loop MAR, β indicates the strength ofthe field which rotates the magnetizations while maintaining themagnetizations of the first and second magnetic layers 15 and 17antiparallel. The strength β approximately corresponds to the coercivityHc2 of the second magnetic layer 17.

[0076] The conditions for rotating the magnetizations of the first andsecond magnetic layers 15 and 17 together while maintaining themagnetizations of the first and second magnetic layers 15 and 17antiparallel are that at least the coercivity Hc2 of the second magneticlayer 17 is smaller than the exchange coupling field Hex1 of the firstmagnetic layer 15, and that the field applied to the magnetic recordingmedium 10 is not larger than the switching field Hsw*.

[0077] Since the sub-hysteresis loop SUR may be regarded as indicatingthe magnetization state of the first magnetic layer 15 as describedabove, a difference between the exchange coupling field Hex1 of thefirst magnetic layer 15 and the switching field Hsw* may be regarded asthe coercivity Hc1 of the first magnetic layer 15. Hence, the switchingfield Hsw* is equal to the sum of the exchange coupling field Hex1 andthe coercivity Hc1 of the first magnetic layer 15 (Hsw*=Hex1+Hc1).

[0078] In the particular case shown in FIG. 3, the coercivity Hc2 of thesecond magnetic layer 17 satisfies the condition Hc2<Hex1, andnaturally, Hc2<(Hex1+Hc1), and Hc2<Hsw*. In this case, the position ofthe switching field Hsw* can be prescribed by use of the exchangecoupling field Hex1 and the coercivity Hc1 of the first magnetic layer15, and used when designing the magnetic recording medium 10.

[0079] By using a recording field within the range MT shown in FIG. 3,which satisfies the relationship Hc2<Hsw*, it is possible to carry outthe recording on the magnetic recording medium 10 while maintaining themagnetizations of the first and second magnetic layers 15 and 17 in theantiparallel state.

[0080] Although the description above is given with respect to thesub-hysteresis loop SUR on the right in FIG. 3, the sub-hysteresis loopon the left is approximately symmetrical, with respect to the origin, tothe loop on the right. Accordingly, a description of the sub-hysteresisloop on the left will be omitted in this specification.

[0081] Next, a more detailed description will be given of the hysteresisloop shown in FIG. 3 with reference to numerical values. In thismodification of the magnetic recording medium 10, the couplingintensifying regions 21 and 22 are provided to intensify the exchangecoupling of the first and second magnetic layers 15 and 17. Accordingly,the exchange coupling field Hex between the first and second magneticlayers 15 and 17 is improved to approximately 5 kOe, and the switchingfield Hsw* is approximately 5.5 kOe.

[0082] The exchange coupling field Hex1 of the first magnetic layer 15is set to be larger than the coercivity Hc2 of the second magnetic layer17, and the coercivity Hc2 of the second magnetic layer 17 and theswitching field Hsw* satisfy the relationship Hc2<Hsw*.

[0083] When the above described relationships are satisfied, it ispossible to always maintain the magnetizations of the first and secondmagnetic layers 15 and 17 in the antiparallel state while a recordingfield is applied from a residual magnetization state indicated by α andthe switching of the magnetizations occurs as indicated α by β in FIG.3. In other words, in the state a (the residual magnetization state),the magnetizations of the first and second magnetic layers 15 and 17 arein the antiparallel state ST2, but when a recording field is applied ina direction opposite to the magnetization of the second magnetic layer17, the magnetization of the second magnetic layer 17 switches to thestate ST3, approximately at the position indicated by β, when therecording field becomes larger than the coercivity Hc2 of the secondmagnetic layer 17.

[0084] In this case, the coercivity Hc2 of the second magnetic layer 17and the exchange coupling field Hex1 of the first magnetic layer 15satisfy the relationship Hc2<Hex1. For this reason, the first magneticlayer 15 is strongly coupled to the second magnetic layer 17, and themagnetization of the first magnetic layer 15 switches simultaneouslywith the magnetization of the second magnetic layer 17, while themagnetizations of the first and second magnetic layers 15 and 17 aremaintained antiparallel. This antiparallel state of the magnetizationsof the first and second magnetic layers 15 and 17 is maintained in theresidual magnetization state, that is, at the position indicated by δ,where the recording field becomes zero.

[0085] In other words, in the above described state where themagnetizations of the first and second magnetic layers 15 and 17 aremaintained antiparallel, the exchange coupling strength (or force) whichacts to maintain the magnetizations of the first and second magneticlayers 15 and 17 antiparallel is larger than the external recordingfield which is applied to the magnetic recording medium 10.

[0086]FIGS. 4A and 4B, respectively, are diagrams showing switching ofthe magnetizations in the present invention of the magnetic recordingmedium 10 and a previously proposed magnetic recording medium which hasbeen proposed in U.S. patent application Ser. No. 09/425,788 describedabove.

[0087] In the case of the present magnetic recording medium 10, theswitching process is completed by switching of the magnetizations onceby a predetermined recording field from state I to state III or viceversa, as shown in FIG. 4A.

[0088] But in the case of the previously proposed magnetic recordingmedium, the switching of the state I to the state III can only berealized via a state II in which the magnetization of the ferromagneticlayer corresponding to the first magnetic layer 15 is parallel to themagnetization of the magnetic layer corresponding to the second magneticlayer 17. In other words, a transition from state I to state II andanother transition from state II to state III are required in order torealize the switching from state I to state III, and a transition fromstate III to state II and another transition from state II to state Iare required in order to realize the switching from state III to stateI.

[0089] Therefore, as may be seen from a comparison of FIGS. 4A and 4B,the present magnetic recording medium 10 can, realize a higher speed ofrecording as compared with the previously proposed magnetic recordingmedium because of the high-speed switching of the magnetizationsdirectly from state I to state III, and vice versa.

[0090] In the present invention, the coupling intensifying region isused to further improve the exchange coupling between the first andsecond magnetic layers 15 and 17. However, the exchange couplingstrength between the first and second magnetic layers 15 and 17 may alsobe adjusted by altering the state of the interface of the materialforming the non-magnetic coupling layer 16. For example, the exchangecoupling strength between the first and second magnetic layers 15 and 17may be adjusted by altering the interface state of Ru which forms thenon-magnetic coupling layer 16. In addition, the exchange couplingstrength between the first and second magnetic layers 15 and 17 may alsobe adjusted and increased by altering the composition and the thicknessof each of the first and second magnetic layers 15 and 17, by alteringthe state of the magnetic grains of each of the first and secondmagnetic layers 15 and 17, or by improving the smoothness of the Ruinterface or the like between the non-magnetic coupling layer 16 and thefirst magnetic layer 15 and/or the second magnetic layer 17. Moreparticularly, the exchange coupling strength between the first andsecond magnetic layers 15 and 17 may be increased by decreasing thethickness of the first magnetic layer 15 and/or the second magneticlayer 17, by increasing the Co-content (or the Co concentration) of thefirst magnetic layer 15 and/or the second magnetic layer 17, or byincreasing the magnetic grain size of the first magnetic layer 15 and/orthe second magnetic layer 17.

[0091] On the other hand, the above described relationship between thecoercivity Hc2 of the second magnetic layer 17 and the exchange couplingfield Hex1 of the first magnetic layer 15 may be maintained, withoutchanging the exchange coupling strength (that is, maintaining theexchange coupling strength approximately constant), by decreasing thecoercivity Hc2 of the second magnetic layer 17. More particularly, thecoercivity Hc2 of the second magnetic layer 17 may be adjusted bychanging the material, the additives and the production process of thesecond magnetic layer 17, so as to change the microstructure, thecrystal structure and the magnetic domain structure. For example, whenforming the second magnetic layer 17 from CoCrPtB, it is possible todecrease the coercivity Hc2 by suppressing the Pt-content of CoCrPtB.

[0092] Furthermore, increasing the coercivity Hc1 of the first magneticlayer 15 is also one method of satisfying the relationship Hc2<Hex1+Hc1.However, if the coercivity Hc1 is increased excessively, there are caseswhere it is no longer possible to maintain the antiparallel state of themagnetizations of the first and second magnetic layers 15 and 17 in theresidual magnetization state. Accordingly, it is also necessary todesign the coercivity Hc1 to be smaller than the exchange coupling fieldHex1.

[0093] One important aspect of the magnetic recording medium 10 of boththe FIG. 1 embodiment and the FIG. 2 modification is that themagnetizations of the first and second magnetic layers 15 and 17 areswitched, while still maintaining the magnetizations of the first andsecond magnetic layers 15 and 17 antiparallel, by applying a recordingfield to the medium 10 that is larger than the coercivity Hc2, and byusing various methods to control the exchange coupling field Hex1 of thefirst magnetic layer 15, the coercivity Hc2 of the second magnetic layerand the coercivity Hc1 of the first magnetic layer 15. In addition, therecording field that is applied to the magnetic recording medium 10 alsoneeds to be smaller than the switching field Hsw*. As a result, it ispossible to carry out a high-speed switching process that switches themagnetizations of the first and second magnetic layers 15 and 17 whilemaintaining the magnetizations of the first and second magnetic layers15 and 17 antiparallel.

[0094] As may be seen from FIG. 3, if the recording field applied to themagnetic recording medium 10 is large when compared to the switchingfield Hsw*, the magnetization of the first magnetic layer 15 becomesparallel to the magnetization of the second magnetic layer 17, which isnot desirable. Accordingly, the maximum value of the recording fieldshould be larger than the coercivity Hc2 of the second magnetic layer17, but smaller than the switching field Hsw*, that is, the maximumrecording field should be set to be within a range between β and γ inFIG. 3. In other words, it is desirable that the maximum value of therecording field from the recording head does not exceed the switchingfield Hsw*.

[0095] Therefore, by controlling the coercivity

[0096] Hc2 of the second magnetic layer 17 and the exchange couplingfield Hex1 of the first magnetic layer 15 so as to satisfy therelationship Hc2<Hex1 and by keeping the recording field from therecording head from exceeding the switching field Hsw*, it is possibleto switch the magnetizations of the first and second magnetic layers 15and 17 while maintaining these magnetizations antiparallel. Unlike thepreviously proposed magnetic recording medium described above, in thepresent invention, state II, in which the magnetizations of the firstand second magnetic layers 15 and 17 become parallel, does not existduring the recording process, and for this reason, the present inventioncan realize high-speed recording. The deterioration of the non-lineartransition shift (NLTS) due to the causes described above will thus notoccur in the present invention. In addition, normal reproduction ispossible even when high-speed reproduction is carried out immediatelyafter recording.

[0097]FIG. 5 is a diagram showing a portion of a recording surface of aso-called patterned medium on an enlarged scale. The patterned medium 30shown in FIG. 5 has a storage capacity per unit area several timesgreater than those of a conventional magnetic recording media. Unlikethe structures of the conventional magnetic recording media, thepatterned medium 30 has unit recording portions 31 which areartificially designed as micro-magnetic recording regions that areformed by a lithography technique, or the like. Boundaries of adjacentunit recording portions 31 are separated on the recording surface of thepatterned medium 30 to thereby realize low noise. Hence, it isunnecessary to use an additive such as Cr to promote segregation andgrain size reduction. For this reason, the magnetic layer may be made ofa material having a small additive content and a large Co-content (Coconcentration). That is, it is possible to use a material that canobtain a large exchange coupling. As a result, it is possible to easilysatisfy the following relationship of the exchange coupling field Hex1of the first magnetic layer 15, in which the exchange coupling fieldHex1 is larger than the coercivities Hc1 and Hc2 of the first and secondmagnetic layers 15 and 17.

[0098] In another embodiment of the magnetic recording medium accordingto the present invention, the present invention is applied to thepatterned medium 30 described above. More particularly, in thisembodiment, each unit recording portion 31 has a stacked structure whichincludes at least the first magnetic layer 15, the non-magnetic couplinglayer 16 and the second magnetic layer 17 which satisfy therelationships of the FIG. 1 embodiment and the FIG. 2 modificationdescribed above. According to this embodiment, it is possible to realizea magnetic recording medium which has a high recording density which isfurther improved and can carry out high-speed recording and reproductionwhich is also further improved.

[0099] In the embodiments and the modification described above, thefirst magnetic layer 15, the non-magnetic coupling layer 16 and thesecond magnetic layer 17 are stacked in this order above thenon-magnetic substrate 11. However, it is of course possible to stackthe second magnetic layer 17, the non-magnetic coupling layer 16 and thefirst magnetic layer 15 in this order above the non-magnetic substrate11. However, in general, it is desirable to arrange the magnetic layerwhich dominates the recording on the side of the magnetic recordingmedium that is closer to the head.

[0100] Next, a description will be given of an embodiment of a magneticstorage apparatus according to the present invention by referring toFIGS. 6 and 7. FIG. 6 is a cross-sectional view showing the basic partsof this embodiment of the magnetic storage apparatus according to thepresent invention, and FIG. 7 is a plan view of the magnetic storageapparatus shown in FIG. 6.

[0101] As shown in FIGS. 6 and 7, the magnetic storage apparatus 40generally includes a housing 43. A motor 44, a hub 45, a plurality ofmagnetic recording media 46, a plurality of recording and reproducingheads 47, a plurality of suspensions 48, a plurality of arms 49, and anactuator unit 41 are all provided within the housing 43. The magneticrecording media 46 are mounted on the hub 45 which is rotated by themotor 44. Each recording and reproducing head 47 is mounted on the tipend of a corresponding arm 49 via the suspension 48. The arms 49 aremoved by the actuator unit 41. The basic construction of this magneticstorage apparatus is known, and a detailed description thereof will beomitted in this specification.

[0102] This embodiment of the magnetic storage apparatus ischaracterized by the magnetic recording media 46. Each magneticrecording medium 46 has the structure of any of the embodiments and themodification of the magnetic recording medium described above inconjunction with FIGS. 1 through 5. In addition, the recording fieldthat is applied to the magnetic recording medium 46 from the recordinghead of the recording and reproducing head 47 is controlled to be bothlarger than the coercivity Hc2 of the second magnetic layer of themagnetic recording medium 46 and smaller than the switching field Hsw*.Of course, the number of magnetic recording media 46 is not limited tothree, and for example, one, two, four or more magnetic recording media46 may be provided.

[0103] The basic construction of the magnetic storage apparatus is notlimited to that shown in FIGS. 6 and 7. In addition, the magneticrecording medium used in the present invention is not limited to amagnetic disk.

[0104] Further, the present invention is not limited to theseembodiments, but various variations and modifications may be madewithout departing from the scope of the present invention.

What is claimed is:
 1. A magnetic recording medium comprising: a firstmagnetic layer having a coercivity Hc1; a second magnetic layer having acoercivity Hc2; and a non-magnetic coupling layer provided between thefirst and second magnetic layers so that the first and second magneticlayers are exchange-coupled and magnetizations of the first and secondmagnetic layers are antiparallel; said first magnetic layer having anexchange coupling field Hex1 that is larger than both said coercivityHc1 and said coercivity Hc2.
 2. The magnetic recording medium as claimedin claim 1, wherein a switching field Hsw* which switches themagnetization of said first magnetic layer to become parallel to themagnetization of said second magnetic layer is set to the sum of saidexchange coupling field Hex1 and said coercivity Hc1.
 3. The magneticrecording medium as claimed in claim 1, wherein a magnetization andthickness product t1Ms1 of said first magnetic layer is smaller than amagnetization and thickness product t2Ms2 of said second magnetic layer,where t1 denotes a thickness of said first magnetic layer, Ms1 denotes amagnetization of said first magnetic layer, t2 denotes a thickness ofsaid second magnetic layer, and Ms2 denotes a magnetization of saidsecond magnetic layer.
 4. The magnetic recording medium as claimed inclaim 1, wherein said coercivity Hc1 is smaller than said coercivityHc2.
 5. The magnetic recording medium as claimed in claim 1, furthercomprising: a coupling intensifying region, provided near a boundary ofsaid first magnetic layer and said non-magnetic coupling layer, whereinsaid coupling intensifying region intensifies an exchange couplingstrength between said first magnetic layer and said second magneticlayer.
 6. The magnetic recording medium as claimed in claim 5, whereinsaid coupling intensifying region is made of a material selected from agroup consisting of Fe, Co, Ni and alloys thereof.
 7. The magneticrecording medium as claimed in claim 1, further comprising: a couplingintensifying region, provided near a boundary of said second magneticlayer and said non-magnetic coupling layer, wherein said couplingintensifying region intensifies an exchange coupling strength betweensaid first magnetic layer and said second magnetic layer.
 8. Themagnetic recording medium as claimed in claim 7, wherein said couplingintensifying region is made of a material selected from a groupconsisting of Fe, Co, Ni and alloys thereof.
 9. The magnetic recordingmedium as claimed in claim 1, further comprising: a first couplingintensifying region, provided near a boundary of said first magneticlayer and said non-magnetic coupling layer, wherein said first couplingintensifying region intensifies an exchange coupling strength betweensaid first magnetic layer and said second magnetic layer; and a secondcoupling intensifying region, provided near a boundary of said secondmagnetic layer and said non-magnetic coupling layer, wherein said secondcoupling intensifying region intensifies the exchange coupling strengthbetween said first magnetic layer and said second magnetic layer. 10.The magnetic recording medium as claimed in claim 9, wherein at leastone of said first coupling intensifying region and said second couplingintensifying region is made of a material selected from a groupconsisting of Fe, Co, Ni and alloys thereof.
 11. The magnetic recordingmedium as claimed in claim 1, which is formed as a patterned medium, andwherein said first magnetic layer, said non-magnetic coupling layer andsaid second magnetic layer are stacked within each of a plurality ofunit recording portions of the patterned medium.
 12. A patterned mediumcomprising: a recording surface; and a plurality of unit recordingportions, provided on said recording surface; having boundaries whichare separated from adjacent unit recording portions, each of saidplurality of unit recording portions having a stacked structurecomprising: a first magnetic layer having a coercivity Hc1; a secondmagnetic layer having a coercivity Hc1; and a non-magnetic couplinglayer provided between said first magnetic layer and said secondmagnetic layer so that said first and second magnetic layers areexchange-coupled and magnetizations of said first and second magneticlayers are antiparallel; said first magnetic layer having an exchangecoupling field Hex1 which is larger than both said coercivity Hc1 andsaid coercivity Hc2.
 13. The patterned medium as claimed in claim 12,further comprising: a coupling intensifying region, provided near aboundary of said non-magnetic coupling layer and at least one of saidfirst and second magnetic layers, wherein said coupling intensifyingregion intensifies an exchange coupling strength between said firstmagnetic layer and said second magnetic layer.
 14. A magnetic storageapparatus comprising: at least one magnetic recording medium; and atleast one head adapted to apply a field to the magnetic recordingmedium; said magnetic recording medium including: a first magnetic layerhaving a coercivity Hc1; a second magnetic layer having s coercivityHc2; and a non-magnetic coupling layer provided between said firstmagnetic layer and said second magnetic layer so that said first andsecond magnetic layers are exchange-coupled and magnetizations of saidfirst and second magnetic layers are antiparallel, said first magneticlayer having an exchange coupling field Hex1 which is larger than bothsaid coercivity Hc1 and said coercivity Hc2.
 15. The magnetic storageapparatus as claimed in claim 14, wherein the field from said head islarger than said coercivity Hc2 and smaller than a switching field Hsw*which switches the magnetization of said first magnetic layer to becomeparallel to the magnetization of said second magnetic layer.
 16. Themagnetic storage apparatus as claimed in claim 15, wherein saidswitching field Hsw* is set to a sum of the exchange coupling field Hex1and said coercivity Hc1.
 17. A magnetic storage apparatus comprising: atleast one magnetic recording medium; and at least one head adapted toapply a field to the magnetic recording medium; said magnetic recordingmedium including: a first magnetic layer; a second magnetic layer; and anon-magnetic coupling layer provided between said first magnetic layerand said second magnetic layer so that said first and second magneticlayers are exchange coupled; wherein, during a recording process, themagnetic field applied to the recording medium is limited to a rangesuch that magnetizations of said first magnetic layer and said secondmagnetic layer are maintained in either a first antiparallel state or asecond antiparallel state, without entering into a parallel state,whereby in said second antiparallel state the magnetizations of saidfirst magnetic layer and said second magnetic layer are reversed, butstill antiparallel, relative to the magnetizations in said firstantiparallel state.
 18. The magnetic storage apparatus as claimed inclaim 17 further comprising: a coupling intensifying region, providednear a boundary of said non-magnetic coupling layer and at least one ofsaid first and second magnetic layers, wherein said couplingintensifying region intensifies an exchange coupling strength betweensaid first magnetic layer and said second magnetic layer.
 19. Themagnetic storage apparatus as claimed in claim 17, further comprising: afirst coupling intensifying region, provided near a boundary of saidfirst magnetic layer and said non-magnetic coupling layer, wherein saidfirst coupling intensifying region intensifies an exchange couplingstrength between said first magnetic layer and said second magneticlayer; and a second coupling intensifying region, provided near aboundary of said second magnetic layer and said non-magnetic couplinglayer, wherein said second coupling intensifying region intensifies theexchange coupling strength between said first magnetic layer and saidsecond magnetic layer.
 20. The magnetic storage apparatus as claimed inclaim 18, wherein the coupling intensifying region includes a materialdispersed within a boundary portion of at least one of said first andsecond magnetic layers.
 21. The magnetic recording medium as claimed inclaim 5, wherein the coupling intensifying region includes a materialdispersed within a boundary portion of at least one of said first andsecond magnetic layers.